A faithful officer is always vigilant towards his duties.

Answer: Option D

Vigilant : keeping careful watch for possible danger or difficulties.
Innocent : not guilty of a crime or offence.
Ignorant : lacking knowledge or awareness in general; uneducated or unsophisticated.
Irresponsible : (of a person, attitude, or action) not showing a proper sense of responsibility.
Careless : not giving sufficient attention or thought to avoiding harm or errors.

Antonym of Vigilant is Careless